Drugs and medicines of different kinds which are to be injected into a patient are usually delivered in vials sealed by a membrane or sealing plug which can be penetrated by the needle of an injection syringe. The substances are in some cases delivered as solutions and in other cases as dry substances, which firstly have to be dissolved in a solvent, e.g. water, which is injected into the vial by an injection springe. In both cases there are pressure balancing problems when pressing liquid into or taking liquid out from the air-tightly sealed vial.